1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a developing apparatus that develops an electrostatic latent image formed on an image bearing member and forms a visible image through an electrophotographic system, an electrostatic recording system, or the like.
2. Description of the Related Art
An image forming apparatus that develops an electrostatic latent image formed on an image bearing member as a toner image through a developing apparatus using a developer including a toner and a carrier configured with a magnetic particle, transfers the toner image onto a recording material, and then fixes the toner image onto the recording material through heating and pressing has been widely used.
The developing apparatus frictionally charges the toner and the carrier by conveying the developer while rotating a screw member to agitate the developer in a circulation path in a developing container. In the developer including the toner and the carrier, as the carrier that is not consumed by image forming continuously circulates while being subjected to friction in the developing container, charging performance of the carrier is gradually lowered. For this reason, in a technique disclosed in Japanese Patent Laid-Open No. S59-100471, a new carrier is supplied to the developing container, and part of the developer conveyed through an outlet formed in a circulation path is caused to overflow and be discharged. As a result, an average charging performance of the carrier of the developer is secured.
Further, a developing apparatus configured such that force acting on a developer in a circumferential direction or an outward diameter direction by rotation of a screw member in a region facing a developer outlet is smaller than other regions has been proposed in Japanese Patent Laid-Open No. 2000-112238. As an embodiment, a configuration in which a blade of a screw member is downsized or omitted in a region facing a developer outlet is described. As a result, it is possible to suppress the developer from being churned up by the blade of the screw member in the developing container facing the developer outlet and discharge only the developer that is truly excessive.
Further, in a technique disclosed in U.S. Patent Application Publication No. 2012/269555 A1, a screw blade of a region facing a developer outlet is omitted, and a rib that agitates or vibrates a developer of a region along the developer outlet with rotation of the screw blade is locally formed in that region. The rib is smaller in diameter than the screw blade, and through this configuration, the developer of the region facing the developer outlet is vibrated. As a result, even in a configuration in which the screw blade of the region facing the developer outlet is omitted or the configuration of suppressing the developer from being churned up by the screw member as in Japanese Patent Laid-Open No. 2000-112238, the developer can be stably discharged by the vibration regardless of fluidity of the developer.
However, a small diameter screw blade or a small diameter rib for vibration is installed around a screw shaft in the portion facing the developer outlet. The developer is not churned up by the small diameter screw blade or the small diameter rib. However, the developer is hoisted by an end portion of the screw blade at the upstream side of the portion in which the screw blade of the region facing the developer outlet is omitted. The developer collides with the small diameter screw blade or the small diameter rib according to an installation phase of the small diameter screw blade or the small diameter rib. As a result, there is a problem in that the developer further is churned up and overflows from the developer outlet (see FIG. 5).
A phenomenon that the developer is churned up by the rib is suppressed by finding an appropriate size of the small diameter screw blade or the small diameter rib. However, the developer that is churned up by the end portion of the screw blade at the upstream side of the portion in which the screw blade is omitted is further churned up by the small diameter screw blade or the small diameter rib. As a result, the churned developer leaks. As described above, the developer that need not be originally discharged from the developer outlet is churned up by the small diameter screw blade or the small diameter rib and overflows. In this case, an amount of the developer in the developing container gradually decreases, and it is difficult to sufficiently coat the surface of the developing sleeve with the developer, and thus density irregularity is likely to occur.
The present invention was made to solve the above problems, and it is desirable to provide a developing apparatus capable of suppressing the developer from being churned up and discharged by a protruding portion arranged in the region facing the developer outlet.